Electron discharge device



J1me 1956 M. B. SHRADER ELECTRON DISCHARGE DEVICE Filed Jan. 15, 1952INVENTOR United States Patent Ofiice ELECTRON DISCHARGE DEVICE MerraldBuren Shrader, Mount Joy, Pa., assignor to Radio Corporation of America,a corporation of Delaware Application January 15, 1952, Serial No.266,589 3 Claims. (Cl. 313299) efiiciency of the tube, since the screencurrent must be subtracted from the available cathode current, and alsocauses heating of the grid structures, with the result that the gridstructures often bow or buckle. This bowing or buckling changes theelectron optics of the tube, or in many cases may cause short circuitsbetween adjacent electrodes of the tube. Thus, excessive control grid orscreen grid current is a limiting factor in determining the maximumallowable power input to the tube.

At very high operating frequencies the time spent by the electrons intraveling from cathode to output electrode must be a small part of thetime required for a single alternation of the high frequency outputcurrent, particularly if high conversion efficiencies are to beobtained. When conversion efliciencies become too low the tube may notdevelop suflicient power to supply its own internal and external circuitlosses, in which case the tube becomes inoperative. This effect limitsthe upper operating frequency of most simple grid controlled electrontubes.

Electron transit time can be reduced by decreasing the distance throughwhich the electron moves or by increasing the voltage of the electrodesbetween which it moves, or both. Since increasing of voltages results ingreater power dissipation, which in itself is undesirable, the mostpractical expedient may be to manufacture tubes having closer spacingbetween electrodes. However, when a conventional screen grid is tested,that is, one having the support rods on the interior surface of itswinding; the support rods limit the close spacing between control gridand screen grid winding.

Two general approaches" have been made towards solving the problem ofexcessive grid current: either the heat caused by the excess gridcurrent must be conducted away from the grid, or the grid current mustbe reduced to an amount where the grid structure will not overheat.Conduction of heat away from the grid structure is the less desirablesolution because in that case the ineiiiciency of the grid operation istolerated so long as the grid structure can withstand the physicaleffects of the high grid current.

With respect to tetrode tubes, screen grid current in prior art tubeshas been reduced to a useful extent by aligning the turns of the controlgrid with the turns of the screen grid, with the result that theelectrons are directed, in the form of pencils or beams, through thespaces between the turns of the screen grid winding. However, the abovementioned prior art practice is not lit) 2,749,469 Patented June 5, 1956entirely satisfactory, mainly because of divergence of the electronbeams or pencils, whereby a substantial amount of electrons do impingeon the turns of the screen grid winding.

The principal object of this invention is to provide a power tube whichwill operate efliciently atvery high frequencies and which will havehigh power output capabilities.

Another object of my invention is to provide an improved grid structureespecially adapted for use in high frequency power tubes, and one whichshall obviate the foregoing and other less apparent disadvantages ofpresent day grid assemblies.

Still another object of this invention is to provide a construction fora power tube, capable of operation at very high frequencies, which willhave efficient grid action and which will be mechanically strong.

A further object of this invention is to provide a tube having highpower output and in which the grid structure is not excessively heated.

A still further object of this invention is to provide a power tubewherein close electrode spacing between control grid and screen grid areobtained, and improved electron beaming action results therefrom.

As applied to a tetrode tube in which screen grid and control grid turnsare in register and in which corresponding grid side support rods shadoweach other, the present invention provides a grid structure whichproduces improved electron beaming, with the result that the screencurrent in the tube is lowered. Improved electron beaming isaccomplished as a result of the side support rods of the screen gridbeing attached to the outer surface of the helical grid winding, forwith such a construction the screen grid winding and the conventionallyconstructed control gn'd may be positioned very close together, and thecontrol grid thus becomes very effective in shielding the screen gridfrom the electron stream. It is also apparent that closer spacingbetween screen grid and control grid result in better transit timecharacteristics, since the electron transit time between cathode andscreen grid, for tetrodes, has a direct bearing on tube performance athigh frequencies.

Besides'advantages of improved electron beaming and improved transittime characteristics, the present invention permits the use of largerand more rugged side support rods for the screen grid, because thesupport rods are positioned in the relatively large space between screengrid and anode.

The invention will be described in greater detail with reference to theaccompanying drawings, in which corresponding parts have the samereference numerals, and in which:

Fig. 1 represents a longitudinal section view of an electron dischargedevice of the tetrode type constructed according to my invention;

Fig. 2 is a transverse sectional view taken along line 22 of Fig. 1;

Fig. 3 represents a transverse section view of an electron dischargedevice of the pentode type constructed according to the invention;

Fig. 4 shows a suitable method for constructing a helical grid for useaccording to the invention.

Referring to Fig. 1, there is shown an electron tube 1 having envelope 3enclosing the operating electrodes of the tube which are spaced fromeach other in conventional manner. A cathode 5, which is shown as anindirectly heated cathode, may however be a filament, or any othersuitable form of emitter, is surrounded by a helical control grid whichconsists of grid wires 7 and support rods 9. Positioned intermediatesaid control grid and a conventional anode electrode 11 is a helicalscreen grid which consists of grid wires 13 and support rods 15. Theturns of the screen grid wires 13 have the same pitch as the turns 7 ofthe control grid and are positioned in registry therewith.

Likewise, as may be seen in Figure 2, the support rods 9 and 15 aresymmetrically located around the circumference of their respectivehelices so that corresponding support rods of the control grid shadowthe corresponding support rods of the screen grid.

Acording to the invention, the support rods 9 of the control grid areattached to the inner surface of the helical grid winding 7, and thesupport rods 15 of the screen grid are attached to the outer surface ofthe helical grid winding 13. This arrangement of the support rods allows(a) the tube grid windings to be spaced very close to each other, whichresults in considerable improved electron beaming action as explainedheretofore, and (b) the support rods, 15, since they are located in therelatively large space between the screen grid and anode 11, may be madelarger and stronger, making a more rugged grid support structure whichis capable of withstanding high temperatures without buckling or ofwithstanding physical shock such as vibrations.

Such an arrangement has been found to provide greatly improved beamingact on, with the result that far less electrons, impinge on the screengrid than occurs when the grid structures are made according to theprior art practice.

Figure 3 shows the top view of a pentode vacuum tube structure in whichthe present invention is used. Control grid winding 7 and screen gridwinding 13 are constructed similarly to those described in Figures -1and 2, but a suppressor grid 21, positioned between screen grid winding13 and anode 11, has been added. It should be apparent that the supportrod construction of control grid 7 and screen grid 13 could be used forany pair of adjacent grids.

Ease of construct ton of a helical grid with externally attached supportrods is illustrated in Figure 4. -A grid wire 7 is wound on a solidmandrel 19, and then support rods 23, held in position by jig 25, arewelded thereon. This arrangement is particularly suitable for use withroller welding machines, since the rollers of the welder can ride on theside rods instead of on the helix wires, thereby eliminating the dangerof burning the individual wires of the grid.

From the foregoing discussion, it will be apparent that the inventionprovides an electrode arangement, in a power tube, which results inimproved efficiency of operation and which provides a more ruggedmechanical structure.

What is claimed is:

1. An electron discharge device including a cathode and anode and a pairof electrically independent grid electrode structures, each having acircular cross sectional configuration and all in concentric and coaxialarray, said grid electrode structures each including a circular array ofside support rods and a helical grid winding for each of said arrays ofsupport rods, each winding comprising conducting elements extendingtransversely of and attached to its support rod array, the winding ofthe inner grid structure being attached to the outer surface of itssupport rod array and the Winding of the outer grid structure beingattached to the inner surface of its support rod array, said windingsbeing close spaced one from another with the spacing between the gridsbeing at least as small as the cross sectional thickness of said supportrods radially of said device, the windings and corresponding supportrods of each of said pair of grid structures being in register with eachother.

2. An electron tube comprising a cathode electrode, a first gridelectrode positioned adjacent to said cathode, a second grid electrodepositioned adjacent to and close spaced from said first grid electrode,and an anode electrode, said cathode and each of said grid electrodesbeing of similar tubular configuration and all of said electrodes beingcoaxially and concentrically arranged, said grid electrodes beingelectrically independent of each other and each including support rodsand a helical winding comprising conducting elements, said conductingelements and corresponding support rods of each grid elecirode being inregister with said conducting elements and support rods of the othergrid, all the support rods of said first grid electrode being positionedbetween its conducting elements and said cathode, and all the supportrods of said second grid electrode being positioned between itsconductive elements and said anode.

3. An electron discharge device including a cathode, a control grid, :1screen grid, and an anode, each having a circular cross sectionalconfiguration and all being coaxial and concentric, said control gridcomprising a circular array of side support rods and a helical gridwinding, said winding comprising conducting elements extendingtransversely of and attached to said support rods about the outersurface of said array of support rods, said screen grid comprising asecond circular array of side support rods and a second helical gridwinding, said second winding comprising conducting elements extendingtransversely of and attached to said second array of support rods aboutthe inner surface of said second array, said windings being close-spacedfrom each other with the spacing between the windings being at least assmall as the cross sectional thickness of said side support rodsradially of said device, the corresponding conducting elements andsupport rods of said control grid and said screen grid being in registerwith each other.

References Cited in the file of this patent UNITED STATES PATENTS2,006,716 Parker July 2, i935 2,310,662 Vogel Feb. 9, 1943 2,447,796Bussey Aug. 24, 1948

